Normally, hydrostatic pressure of the drilling-fluid column is greater than pressure of formation fluids, thus preventing flow of formation fluids into the wellbore. When hydrostatic pressure drops below formation-fluid pressure, formation fluids can enter the well. If this flow is small, causing a decrease in density (mud weight) as measured at the surface, the drilling fluid is said to be "gas cut", "salt-water cut", or "oil cut" as the case may be. When a noticeable increase in mud-pit volume occurs, the typical prior art method of gas influx detection, the event is known as a "kick" . An uncontrolled flow of formation fluids into the wellbore and up to the surface is a "blowout".
As long as hydrostatic pressure controls the well, circulation is accomplished by using a flowline, or the well may be left open while the bit is removed. If a kick occurs, blowout-prevention equipment and accessories are needed to close the well. This may be done with an annular preventer, with pipe rams, or with master (blind) rams when the drill pipe is out of the hole.
In addition, means are necessary to pump drilling fluid into the well and to allow controlled escape of fluids. Injection is accomplished either down the drill pipe or through one of the kill lines, and flow from the well is controlled by a variable orifice or choke attached to a choke line. Choke lines are arranged so that well effluent can be routed to either a reserve pit where undesired fluid is discarded, or to a mud/gas separator, degasser, and mud pit where desired fluid is degassed and saved. By using this equipment, the low-density fluids are removed and replaced with a higher-density fluid capable of controlling the well.
As mentioned above, kick detection while drilling in the past has typically been indicated by observing and monitoring the mud return flow rate and/or mud pit volume. Accordingly, most rigs which use drilling mud to control the pressure in the borehole have some form of pit-level indicating device to indicate a gain or loss of mud. A mud pit-level indicating and recording device such as a chart is usually located in a position so that the driller can see the chart while drilling is occurring. When a kick occurs, the surface pressure required to contain it will largely depend upon closing in the BOP's quickly and retaining as much mud as possible in the well.
A flow meter showing relative changes in return-mud flow has also been used as an early warning device, because mud hold-up in solids control devices, degassers, and mixing equipment affects average pit-level. Such fluctuations in pit-level due to such factors recur periodically during drilling and may occur simultaneously with a kick. When such conditions are present, a return-flow rate may be the first indication of a kick.
To determine kicks as early as possible while drilling, the driller typically uses instantaneous charts of average volume of the mud pit, mud gained or lost from the pit, and return-flow rate. Preferably, the pit volume and return flow rate is recorded on the drilling floor so that trends can be established. As soon as an unexpected change in the trends of such parameters occurs, a driller checks for a kick condition.
Because a kick can lead to a blowout with possible disastrous results to the well, prior attempts have been made to obtain information as to a gas influx into the borehole before such influx affects pit mud volume or return flow rate.
For example, U.S. Pat. Nos. 4,733,233 to Grosso and Feeley and 4,733,232 to Grosso describe a technique by which a pressure transducer at the surface senses annulus acoustic variations in the returning mud flow and another pressure transducer at the surface senses drill string acoustic variations in the entering mud flow.
In the '232 patent, a downhole "wave generator" produces an acoustic signal in the sonic range. The signal is measured at the surface in the drill string and in the annulus. Changes in the measured difference between amplitude and phase of the annulus and drill string signals are said to indicate that fluid influx into the annulus has occurred.
In the '233 patent, a downhole MWD transmitter produces a train of pulses in the sub-sonic or sonic frequency range. The pulse trains are sensed at the surface in the annulus and in the drill string or standpipe with pressure transducers. A change in the amplitude of the annulus signal where no change occurs in the amplitude of the drill string signal is used to indicate the presence of a borehole fluid influx. A change in phase angle between the surface received annulus signal and the surface received drill string signal is also used to indicate a borehole fluid influx.
Such amplitude and phase comparisons of annulus and drill string surface signals which travel upwardly through the annulus and drill string respectively from an MWD communication transmitter have been found to be inaccurate under many circumstances. Amplitude comparisons of such signals are difficult in the real world environment of a drilling rig and deep borehole due to noise which is simultaneously measured in the annulus and drill string and due to variations between annulus and drill string mud temperature. The phase difference between the annulus and drill string signals is inherently ambiguous because the phase of the annulus signal may be less than or greater than 360.degree. (2.pi.) from that of the drill string.
The '233 patent suggests that a correlation function may be obtained between the annulus and drill string signals and that such signals have a fixed time relationship .tau.. The patent further suggests that characteristics of the annulus and drill string may be precisely determined on a continuous basis while drilling and that if characteristics of the annulus and drill string signals are disturbed in excess of a predetermined limit, an alarm may be energized. Unfortunately, a direct correlation process as suggested by the '233 patent has been found to be useless without an explanation as to how the annulus and drill string signals are to be "conditioned" prior to the correlation process.
Another technique for determining fluid influx into the borehole while drilling is disclosed in U.S. Pat. No. 4,273,212. This patent discloses energizing a transducer to propagate an acoustic signal down the annulus between the borehole and the drill string. A receiver is provided to receive reflected acoustic energy at the surface. Such acoustic energy is reflected from the bottom of the hole and also from the interface between drilling fluid in the annulus and fluid influx. This technique is believed not to be feasible in a real drilling rig environment due to the difficulty of distinguishing reflections from the bottom of the hole, reflections from discontinuities in borehole casing, and reflections from true mud density changes caused by fluid influx. Moreover, the technique of the '212 patent suffers from a practicality viewpoint because it requires circulation through the choke.
In light of the above, a major object of the present invention is to provide a practical fluid influx system for an operating rotary drilling rig.
Another object of the invention is to provide a practical way during drilling to determine fluid influx into a borehole by comparing transit time to the surface via the annulus and with that of the drill string of an MWD communication mud pulse train.
Another object of the invention is to provide a practical way of determining fluid influx into a borehole while it is being drilled by comparing transit time to the surface via the annulus with that of the inside of the drill string of drilling noise generated by the interaction between the bit and the rock.
Another object of the invention is to provide a practical way of determining fluid influx into a borehole while it is being drilled from a standing wave analysis of the magnitude and phase of periodic acoustic signals caused by the mud pumps of the drilling rig.
Another object of the invention is to provide a practical way of determining fluid influx into a borehole while it is being drilled from the analysis of the total transit time of mud pump beats down the drillstring and up in the annulus.
Another object of the invention is to simultaneously require a fluid influx determination (1) from a mud pump standing wave analysis (2) from a mud pump beat propagation analysis and (3) from a transit time analysis of an MWD communication mud pulse train or a downhole noise source associated with the interaction between the bit and the formation before a fluid influx alarm is provided to a driller.
Another object of the invention is to provide apparatus for informing a driller as to the location and size of a gas slug that has entered the borehole.